It is well known that adsorptive separation is one of the separation methods widely adopted in the chemical industry, especially in petrochemical industry. For quite some time, this method has been adopted to separate a component which is difficult to be separated by other means from a mixture of feed containing various components.
In the prior art , there are abundant patent literatures describing methods for separating one kind of hydrocarbon from the other isomers. For instance, methods for separating para-isomers of monocyclic aromatics substituted by dialkyl group from other isomers, especially for separating paraxylene from other xylene isomers by employing a specific zeolite molecular sieve adsorbent to preferably adsorb para-isomers have been published in literatures of U.S. Pat. No. 3,626,5020, U.S. Pat. No. 3,663,638, U.S. Pat. No. 3,665,046, U.S. Pat. No. 3,700,744, U.S. Pat. No. 3,686,342, U.S. Pat. No. 3,734,47, U.S. Pat. No. 3,394,109, U.S. Pat. No. 3,997,620, CN1022622, CN1022826, CN10493294, CN1051549A, CN1064071 and CN1047489A in which benzene, toluene, chlorobenzene, fluoro-aromatics, halogen toluene, para-dialkylbenzene,diethyltoluene, and tetraline are respectively recommended as the desorbent depending on the composition of mixture of the feed.
A process of adsorptive separation may be effected both on a fixed bed or a moving bed , preferably on a simulated countercurrent moving bed system. For example, a simulated countercurrent moving bed system has been adopted for adsorptive separation in U.S. Pat. No. 2,985,589, U.S. Pat. No. 3,268,604 and U.S. Pat. No. 3,268,605 while a rotary valve for the system of simulated countercurrent moving bed has been disclosed in U.S. Pat. No. 3,040,777 and U.S. Pat. No. 3,422,848. Some drawbacks in the prior art , the objects of this invention and the scheme of settlement will be further explained hereinafter with reference to the accompanying drawing.
The drawing is a principle depiction of a continuous adsorptive separation system of a simulated countercurrent moving bed.
Referring to the drawing, a adsorptive separation system comprises four zones, consecutively as adsorption zone, purification zone, desorption zone and buffer zone. In the drawing, F represents a feed stream containing a selectively adsorbed component A and a relatively less adsorbed component B, D represents a desorbent stream, E represents an extract, i.e. a stream of desorbent containing the selectively adsorbed component A, R represents a raffinate, i.e. the remaining stream containing the relatively less adsorbed component B after desorption, H.sub.(in) and H.sub.(out) respectively represent an inflowing and an outflowing stream enriched in said desorbent for the primary flush stream for the lines connecting the adsorptive beds, X represents the secondary flush stream for the lines connecting the adsorptive beds, and M represents the simulated adsorbent moving direction shifted by the rotary valve. Zone I is between F and R wherein the charged feed contacts countercurrently with the adsorbent, and the selectively adsorbed component A shifts from the feed stream into the pores of said adsorbent, displacing the desorbent D from the pores at the same time. Thus, zone I is defined as adsorption zone. Zone II is between F and E. For the reason that the adsorbent adsorbs selectively adsorbed component A and a little amount of relatively less adsorbed component B as well, in zone II said adsorbent contacts with the stream containing only A and D coming from upstream of zone II, relatively less adsorbed component B is displaced gradually from the pores by selectively adsorbed component A and the desorbent D by means of appropriate adjustment of flow velocity of the stream in the zone. As the adsorbent has a stronger adsorption selectivity to component A than to component B, component A will not be completely displaced at the same time and thus will get purified in the zone. Zone II is consequently defined as purification zone. Zone III is between E and D wherein pure D contacts with the adsorbate purified in zone II and displace A from said adsorbent pores, thus this zone is defined as desorption zone. Zone IV is between D and R wherein the flowrate of D is defined so that D is made to flow upwards in the zone under flow control so as to prevent component B from getting into the stream in zone III to contaminate the extract. Thus, this zone is defined as buffer zone .
In operation, switching equipment, e. g. rotary valve, etc. is employed to recycle the inflowing and outflowing streams and shift the four zones in turn to realize simulation of adsorbent moving. During rotary valve switching, it is necessary to flush the residue out of the adsorptive bed connection lines to ensure purity and recovery of the purified component. Locations of H.sub.(in) , H.sub.(out) and X are shown in the drawing and thus zones II and III are further divided into three more zones of II', II" and III'.
With regard to flowrate of H.sub.(in) and H.sub.(out), on one hand if the flowrate is set too small, the residue in the lines cannot be flushed away, which will subsequently affect the product purity and recovcry; on the other hand, if the flowrate is too big, when the flush stream enriched in desorbent is drained through adsorptive beds after flushing the connection lines, adsorptive space of the adsorbent will be occupied by the desorbent in the flush stream , whereby weakening the adsorptive power of said adsorbent to the selectively adsorbed component, which will also result in the decreased purity and recovery of the selectively adsorbed component .
In the prior processes, the set flowrate W.sub.H(in) and W.sub.H(out) of primary flush stream to each adsorptive bed connection line is calculated based on the following formula (I): EQU W.sub.H(in) =W.sub.H(out) =2V.sub.L /T (I)
wherein:
V.sub.L - - - volume of the longest connection line,m.sup.3
T - - - time interval of rotary valve switching, h
Based on volumetric equilibrium of the adsorber, the following formulas can be adopted for calculating flowrate in each zone; EQU W.sub.I =W.sub.H +W.sub.S +W.sub.F EQU W.sub.II' =W.sub.H +W.sub.S EQU W.sub.II" =W.sub.S -W.sub.X EQU W.sub.III =W.sub.S +W.sub.E -W.sub.X EQU W.sub.III' =W.sub.S +W.sub.E +W.sub.H -W.sub.X EQU W.sub.IV =W.sub.D +W.sub.H -W.sub.D -W.sub.X
wherein:
W.sub.H - - - flowrate of the primary flush stream
W.sub.X - - - flowrate of the secondary flush stream
W.sub.F - - - flowrate of the feed stream
W.sub.E - - - flowrate of the extract stream
W.sub.D - - - flowrate of the desorbent stream
W.sub.S - - - flowrate of the stream specified in zone II
In an actual system, however, because of the different position of each bed spaced in the adsorber and different distance to the rotary valve, the volume of each line connecting bed with rotary valve varies a lot. For this reason, the calculation of the flowrate of primary flush stream for each line based on formula (I) in prior proeess will result in higher flush stream which will hence decrease the purity and recovery of the product.